Past Awards

FY 2023 | 2 – Respond | 23-10-05

Oceanography of Cook Inlet Tide Rips

Tyler Hennon, University of Alaska-Fairbanks (UAF)
Contract Term: 04/01/23 - 03/31/25
Award: $

Interaction of tidal and buoyancy currents with certain bathymetry features cause
persistent, strongly convergent and divergent shear zones, called tide rips, at relatively
fixed locations along Cook Inlet. Though relatively small-scale features (~100s m), these
convergence zones influence fish movements, concentrate debris and ice, cause east-towest variations in the southward current velocities and enhance vertical exchange.
Downward vertical velocities of up to 20 cm s-1 have been measured in these
convergence zones, fast enough to temporarily and locally overcome the buoyancy of oil
(Johnson et. al. 2000). Despite the fact that they might be one of the most important
features affecting the fate and transport of spilled oil, the diurnal and fortnightly
variability of the convergence zones are not well described.
The investigators of this proposal recently received a three-year award from the
Bureau of Ocean Energy Management (BOEM) to, in part, conduct a field campaign
focused on collection of in situ observations of tidal rip currents in Cook Inlet (projected
for summer of 2024). The objectives of the fieldwork are to 1) Conduct repeat transects
across rip currents and collect vertical profiles of temperature, salinity, and velocity; and
2) Perform repeated deployments and recoveries of surface-drogued drifters to evaluate
the convergence/divergence near known rip current zones. In support of this project, the
Cook Inlet Regional Citizens Advisory Council (CIRCAC) has committed $80,000.
This proposal seeks OSRI funding to augment our field program and expand its
capacity to characterize the thermohaline structure of Cook Inlet tidal rips. Support from
CIRCAC will provide funds to charter a lightweight oceanographic vessel for
approximately two weeks to collect observations for a full spring neap cycle. The vessel
will be stocked with ~10 suface-drogued drifters and equipped with an acoustic Doppler
current profiler (ADCP) to quantify surface and subsurface currents. Practically,
observations of the thermohaline structure, and by extension buoyancy forcings, are
challenging within Cook Inlet, particularly in rip zones where currents can reach 8+
knots. Traditional deployment methods (e.g. a conductivity-temperature-depth (CTD)
rosette or a towed CTD chain) are ill-suited for such an extreme current environment,
which will necessitate making significant concessions in the resolution of the
thermohaline structure across rips. For example casts may need to be collected on either
side of the rip zone, missing core features.
Additional support from OSRI will allow us to acquire a winch specifically
designed to operate in strong currents and enhance our capacity to resolve the
thermohaline structure of Cook Inlet tidal rip zones. The RapidCAST winch (distributed
by Teledyne Marine) enables vertical CTD profiles at vessel speeds (through water) of up
to 12 knots, which will allow us to perform continuous CTD transects across strong rip
zones. The winch is also designed for rapid CTD down/upcast speed, such that in the
relatively shallow waters of Cook Inlet the lateral spacing between successive profiles is
expected to be as little as 10s of meters.
Between the leveraged support of CIRCAC and the support of OSRI to acquire a
dynamic winch well-suited to sampling rip currents, we expect to generate a dataset with
unprecedented spatiotemporal resolution of currents and thermohaline structure within
Cook Inlet tidal rip zones. This will not only greatly improve our ability to quantify the
lateral dispersive nature of rip currents, but also evaluate the dynamics of buoyancydriven currents that may be impo